Dual absorption sulfuric acid process

ABSTRACT

A dual absorption sulfuric acid process is provided, with intermediate scrubbing of the process gas stream to remove sulfur trioxide, in which the residual gas stream after intermediate scrubbing is heated to a temperature which is about 10* C. to 50* C. higher than the temperature of the initial hot process gas passed into the first stage of catalysis. The sulfur dioxide content of the heated residual gas stream is thus catalytically oxidized to sulfur trioxide using reduced amounts of catalyst.

United States Patent Connor et a].

[54] DUAL ABSORPTION SULFURIC ACID PROCESS [72] Inventors: John M.Connor, New York, N.Y.; Krikor D. Gureghian, Teaneck, NJ.

[73] Assignee: Chemical Construction Corporation, New

York, NY.

[22] Filed: Sept. 24, 1969 [21] Appl. No.: 860,519

[52] [1.8. Cl ..23/l68, 23/176 [51] Int. Cl. ..Clb 17/76 [58] FieldofSearch ..23/168, 175,176

[56] References Cited UNITED STATES PATENTS 1,789,460 1/1931 Clark..23/176 3,350,169 /1967 Rinckhofi' ..23/168 3,404,956 10/1968 Drechselet a1 23/ 168 Apr. 4, 1972 3,525,587 8/1970 Browder... ..23/1es3,536,446 10/1970 Maurer ..23/168 OTHER PUBLICATIONS Djecker (editor),The Manufacture of Sulfuric Acid pages 164- 168 (Reinhold 1959) PrimaryExaminer-Oscar R. Vertiz Assistant Examiner-Charles B. Rodman Art0rneyJ.L. Chaboty [5 7] ABSTRACT A dual absorption sulfuric acid process isprovided, with intermediate scrubbing of the process gas stream toremove sulfur trioxide, in which the residual gas stream afterintermediate scrubbing is heated to a temperature which is about 10 C toC. higher than the temperature of the initial hot process gas passedinto the first stage of catalysis. The sulfur dioxide content of theheated residual gas stream is thus catalytically oxidized to sulfurtrioxide using reduced amounts of catalyst.

6 Claims, 3 Drawing Figures Patented April 4, 1972 3,653,823

3 Sheets-Sheet 1 JOHN M. CONNOR KRIKOR D. GUREGHIAN INVENTORS.

A G E N T Patented April 4, 1972 v 3553,82

3 Sheets-Sheet 2 JOHN M. CO-NNOR KRIKOR 0. GUREGHIAN INVENTORS,

BY 7. (2a

AGENT Patented April 4, 1972 3,s53,s2

S Sheets-Sheet 5 JOHN M. CONNOR KRIKOR Dv GUREGHIAN 1N VENTORS AGENT IDUAL ABSORPTION SULFURIC ACID PROCESS BACKGROUND OF THE INVENTION Fieldof the Invention The invention relates to dual absorption processes forproduction of sulfuric acid, in which the initial hot process gas streamcontaining sulfur dioxide and oxygen is passed through one or morestages of catalysis for conversion of a portion of the sulfur dioxide tosulfur trioxide, the resulting gas stream containing sulfur trioxide,residual sulfur dioxide and residual oxygen is cooled and scrubbed withconcentrated sulfuric acid or oleum to remove sulfur trioxide, thescrubbed gas stream is heated and subjected to further catalysis toconvert residual sulfur dioxide to sulfur trioxide, and the resultinggas stream is cooled and scrubbed with concentrated sulfuric acid oroleum, so as to produce an effluent tail gas stream substantially freeof sulfur oxides. The invention is also applicable to existing sulfuricacid production facilities in which objectionable amounts orconcentrations of sulfur dioxide are present in the effluent tail gas.Such existing facilities may be converted to dual absorption processesaccording to the present invention, with resultant diminished dischargeof sulfur dioxide to the atmosphere and curtailment of air pollution.

Description of the Prior Art Sulfuric acid is generally produced by theoxidation or combustion of a sulfur-containing material such aselemental sulfur, hydrogen, sulfide, pyrites, sludge acid or the like,to produce a process gas stream containing sulfur dioxide and oxygen,which is passed at elevated temperature through one or more stages ofcatalysis in contact with a platinum or vanadium pentoxide catalyst, soas to oxidize sulfur dioxide to sulfur trioxide. The resulting gasstream is cooled and scrubbed with concentrated sulfuric acid or oleum,so as to produce further sulfuric acid or fortified oleum of greatersulfur trioxide concentration. A residual tail gas of law sulfur dioxidecontent is discharged to the atmosphere. Due to more stringent airpollution regulations promulgated in recent years, dual absorptionsulfuric acid processes as described supra, which discharge a tail gashaving negligible sulfur oxides content, have been developed. Dualabsorption sulfuric acid processes are described in U.S. Pat. Nos.3,432,263; 3,350,169; l,789,460 and U.S. Pat. application No. 670,551filed Sept. 26, 1967 and now issued as U.S. Pat. No. 3,536,446.Improvements in conventional sulfuric acid are described in U.S. Pat.Nos. 3,147,074; 3,455,652; 3,172,725

and U.S. Pat. application No. 626,489 filed Mar. 28, 1967 and now issuedas U.S. Pat. No. 3,475,120.

SUMMARY OF THE INVENTION In the present invention, an improved processof design and operation of a dual absorption sulfuric acid plant hasbeen developed. The improvement arises from the fact that it has beendetermined that by operating the catalyst stages after the firstabsorption stage at a temperature which is higher than normal, typicallywith a difference above normal of about 10 C. to C, the catalystquantity required is appreciably reduced. This leads to a plant designwhich preferably has two stages of conversion beforethe first absorptionstage and one or two further stages after the first absorption stage.

After the primary absorption stage of a dual absorption sulfuric acidplant, the ratio of oxygen to sulfur dioxide is very high compared tothe values usually encountered in single absorption plants. In thesecircumstances, the composition of this gas at its thermodynamicequilibrium point varies only slightly with temperature in the range 425C. to 480 C. and consequently within this range thermodynamicequilibrium effects on the rates of reaction are comparatively small,and the effect of temperature becomes much more important. For instance,a gas containing 1 percent sulfur dioxide and 6.5 percent oxygensupplied to a catalyst at 425 C. could theoretically come to equilibriumat about 460 when it contain approximately 0.032 percent sulfur dioxide.If the same initial gas was supplied to the catalyst at a highertemperature of 453 C. the equilibrium temperature would be 485 C. andthe final sulfur dioxide content would be only slightly greater, 0.045percent sulfur dioxide content at equilibrium. The quantity of catalystrequired to reach any given final sulfur dioxide content would be muchless at the higher initial temperature.

It has also been determined that 2/2 dual absorption systems, with twoconversion stages before primary absorber and two more after the primaryabsorber, are more efficient in catalyst usage than 3/1 systems.Typically a 2/2 system using higher feed temperatures into the stagesafter primary absorption decreased catalyst volume to 65 percent of theamount required for the more usual lower temperature 3/1 process, forthe same overall end conversion. The high temperatures 2/2 dualabsorption system has the additional advantages of lower pressure dropthrough the system due to decreased depth of catalyst beds after theprimary absorber. The reduced pressure drop lowers overall powerconsumption. The new system also reduces heat exchanger surface due toan increased temperature difference between the tube side inlet andshell side outlet streams of the primary absorber heat exchanger, ascompared to a conventional 3/1 dual absorption system in which threecatalytic stages are provided before primary absorption and one stageafter.

From a design viewpoint, a dual absorption system is quite differentfrom a once-through three or four bed converter. The once-through systemis basically limited to one equilibrium conversion vs. temperaturecurve, calculated from the feed gas composition to the first bed. Thehigher the gas temperature, the lower the equilibrium conversion. Hence,it is necessary to cool the gas in between beds to a temperature at orclose to the initial bed feed temperature. Once-through converter gashas to under-go cooling prior to succeeding beds to the same temperatureof 425 C. to 440 C. for all the beds involved. However, dual absorptionconverters and processes have two equilibrium conversion CURVES. Thecurve prior to the first or primary absorber is similar to theonce-through curve, differing only because of the increase in sulfurdioxide concentration, hence the same adverse high temperature effectson conversion. However, the equilibrium conversion curve for the gasafter the first intermediate or primary absorber, when on the same scaleas the original curve, is above the first curve and shows only a smallreduction in conversion with increased temperatures. Hence, temperatureeffects on equilibrium conversion are not as significant. Thus, in adual absorption system, it is possible to take advantage of using higherfeed temperatures in the catalyst stages after primary absorption andthereby take advantage of the higher reaction rates thus obtained withresultant reduced catalyst requirements. It was determined that thetypical scheme of operation for sulfur burning dual absorption plants isa fourstage converter, having two catalyst stages before primaryabsorption and two after, feed temperatures being about 420 C. 430 (2.,455 C. and 455 C. for the first, second, third and fourth stages,respectively. None of the combinations of three to one dual absorptionschemes proved as efficient as the arrangement involving two stagesbefore primary absorption and two stages after.

There are a large number of existing once-through sulfuric acid plantsdesigned originally for about 96 percent conversion and probablycurrently operating below this conversion rate, with resultant excessivesulfur dioxide discharge. These plants will not be permitted to continueoperation under existing and pending air pollution control legislation.Some of the existing plants are in good condition and-completereplacement is not justified.

The dual absorption system as applied to existing oncethrough sulfuricacid plants means that the sulfur trioxide formed in the existing two orthree stages of conversion is absorbed, the tail gas is reheated, andthe heated tail gas is passed through one or two additional newconversion stages before final absorption. Systems vary in the way theheat required for reheating the primary absorber tail gas is obtained,but in almost all cases this reheating requires some heat from the firstconversion stages. If it were not for this reheat requirement, it wouldbe a simple matter to modify an existing facility by adding a secondconversion and absorption stage to any existing once-through sulfuricacid plant. In an existing acid plant, the relatively high temperatureheat available in the gas discharged from the first conversion stage isused either to reheat inlet gas which has been cooled below reactiontemperature in the burner gas steam boiler, or to generate steamdirectly in a waste heat boiler. The second stage heat is most usuallyremoved by internal coils in the converter, through which air iscirculated and heated, with the heated air being used in the sulfurburning furnace or wasted, or the internal coils may contain circulatingboiler feed or condensate water which is heated and vaporized to produceusable superheated steam. The only other source of heat is the hotefiluent gas from the sulfur furnace.

Some of the heat required to reheat the tail gas after the firstabsorption tower can be obtained from a feed/efiluent exchanger on thesecondary converter, but because of the lower temperatures and the needfor reasonable temperature approaches in the exchangers, as well as thenecessity for initially heating the plant, some of this heat must beobtained from a high temperature source, such as the primary convertergas or the sulfur burner gas.

When employing the primary converter gas as a high temperature heatsource, the gas from the stack of the existing plant is pressurized by ablower and heated in a first heat exchanger by the gas leaving thesecondary converter, and then further heated in a second exchanger usingthe gas from the first stage of the primary converter. The basic schemeincludes a converter exchanger rather than a second boiler. On the hotgas side, the new exchanger is connected in parallel with the converterexchanger of the original plant. As the heat requirement of the newplant is obtained at the expense of the converter inlet gas of the oldplant, this must be made up by opening the furnace gas steam boilerbypass damper, which in turn will reduce the steam production by theequivalent of the heat removed from the system. The new converter may bea single or two-stage unit, and the remainder of the added new plantincludes the secondary absorption tower with its own acid circulationsystem and acid cooler, and connection to the acid system of theexisting plant. In most instances, it is possible to use the convertergas-to-gas heat exchanger of the existing plant for the new duty,connecting the new converter inlet stream in place of the originalconverter inlet stream. The process gas inlet to the old or existingconverter would then be taken directly from the furnace gas steam boileroutlet, and the additional heat would be obtained by boiler bypass. Thismodification results in a lower initial cost but a greater loss of steamproduction.

When employing the sulfur burning furnace gas as a high temperature heatsource, the high temperature heat for final heating of the secondaryconverter inlet gas is obtained by mixing the tail gas, which may firstbe preheated by heat exchange with the secondary converter effluent gas,with hot gas produced at about 800 C. to 1,000 C. by the sulfur burningfurnace. As this gas usually contains at least percent sulfur dioxideand it bypasses the first converter and absorber, it usually becomesnecessary to use a two-stage secondary converter to obtain the desiredhigh overall sulfur dioxide conversion of 99.5 percent or higher. Theadvantage of this schemeis that it requires only one gas connection lineto the existing plant, in addition to the inlet line from the stack, andthis gas connection line for transferring hot sulfur burner gas forheating purposes although very hot need not be very large. An additionaladvantage arises because, since the hot gas addition bypasses the wholeof the existing or base plant except the drying tower, furnace andblower, it is possible to increase the total plant capacity if theseunits will accommodate greater throughput. If the blower capacity isfully employed in the existing plant, the blower of the additional plantmay be adjusted in capacity to overcome this restriction and permitgreater overall throughput.

It is an object of the present invention to provide an improved processfor producing sulfuric acid.

Another object is to provide an improved dual absorption sulfuric acidprocess, in which the dioxide-containing process gas stream trioxide, isa partially oxidized to sulfur trioxide,

scrubbed with concentrated sulfuric acid to remove sulfur dioxide in thetail gas discharged from a sulfuric acid process.

An object is to reduce the size of heat exchangers required for a dualabsorption sulfuric acid process.

These and other objects and advantages of the present invention willbecome evident from the description which follows.

DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS Referring now tothe drawings,

FIG. 1 is a flowsheet of the process of the present invention in apreferred embodiment as applied to a new sulfuric acid productionfacility;

FIG. 2 shows an embodiment of the invention as applied to an existingonce-through facility, in which reheat of the tail gas is accomplishedby heat exchange with hot primary converter gas, and

FIG. 3 illustrates an alternative embodiment of the invention as appliedto an existing once-through facility, in which reheat of the tail gas isaccomplished by direct injection of hot gas from the sulfur burningfurnace.

Referring now to FIG. 1, sulfur stream 1 is burned with predried airstream 2 in sulfur burning furnace 3, which is generally arefractory-lined chamber; The resulting hot furnace gas stream 4discharged from unit 3 is typically at a temperature in the range of 800C. to l,000 C. and usually contains about 8 percent to 14 percent sulfurdioxide content by volume, together with excess oxygen and inerts suchas nitrogen. Stream 4 is cooled to the proper temperature for initialcatalytic oxidation of sulfur dioxide to sulfur trioxide, by passingstream 4 through waste heat steam boiler 5. Boiler feed water orcondensate water stream 6 is circulated through unit 5 in indirect heatexchange with stream 4, and the resultant generated and usuallysuperheated steam is removed via stream 7.

The cooled process gas stream 8 discharged from unit 5 is now at asuitable temperature for initial catalysis, generally in the range ofabout 400 C. to 450 C. Stream 8 is passed into the upper end ofcatalytic converter 9, and the process gas flows downwards throughcatalyst bed 10, which is generally a suitable catalyst for thecatalytic oxidation of sulfur dioxide to sulfur trioxide, such asplatinum, vanadium oxide or the like.

The reaction is exothermic, and the hot gas discharged below bed 10 mustbe cooled before further catalysis, by diverting the gas above partition11, and removing the hot partially converted gas from unit 9 via stream12, which is at a temperature generally in the range of 500 C. to 630 C.Stream 12 is cooled to a suitable temperature for further catalysis, bypassing stream 12 through waste heat steam boiler 13. Boiler feed orcondensate water stream 14 is circulated through unit 13 in indirectheat exchange with the hot process gas stream, and generated steam isremoved via stream 15. The cooled process gas stream 16 is dischargedfrom unit 13 at a temperature typically in the range of 400 C. to 450C., and stream 16 contains sulfur trioxide, residual sulfur dioxide andoxygen, and inerts. Stream 16 is passed back into unit 9 below partition11, and the process gas stream flows downwards through catalyst bed 17,which is generally similar in configuration and function to bed 10described supra. Further catalytic oxidation of sulfur dioxide to sulfurtrioxide takes place in bed 17, and the resulting hot gas mixturedischarged below bed 17 at a temperature typically in the range of 450C. to 550 C. now typically contains in the range of about 6 percent to12 percent sulfur trioxide content by volume, 0.5 percent to 4 percentresidual sulfur dioxide content, residual oxygen and inerts. The hot gasis diverted below bed 17 by partition 18, and removed from unit 9 abovepartition 18 via stream 19, which is now processed in accordance withone embodiment of the present invention.

Stream 19 is now passed through process gas-to-gas heat exchanger 20,and is cooled while reheating sulfur trioxidefree process gas byindirect heat exchange in accordance with the present invention. Thecooled process gas stream 21 discharged from unit 20 is passed into gasscrubbing tower 22 below gas-liquid contact section 23, which mayconsist of a bed of spheres, saddles or rings packing or the like, or aplurality of bubble cap or sieve trays, or other suitable gas-liquidcontact means. The rising gas phase in section 23 is scrubbed by astream 24 consisting of concentrated sulfuric acid typically containingin the range of about 93 percent to 99 percent sulfuric acid content byweight. In other instances, stream 24 may consist of oleum. In any case,stream 24 is passed into unit 22 above section 23, and flows downwardscountercurrent to the rising gas phase, and selectively absorbs sulfurtrioxide from the gas phase in section 23, with resultant formation offurther sulfuric acid or enriched oleum in solution. The resultantliquid phase collects in the bottom of unit 22 and is removed via stream25, which is generally cooled and passed to product utilization. In mostinstances, a portion of stream 25 will be diluted with water or dilutesulfuric acid and the diluted acid or oleum will be recycled via stream24.

The residual gas stream 26 removed from unit 22 above section 23 nowcontains residual sulfur dioxide and oxygen, together with inerts suchas nitrogen, and stream 26 is now reheated in accordance with thepresent invention, by passing stream 26 through heat exchanger and inindirect heat exchange with stream 19. The resulting reheated processgas stream 27 discharged from unit 20 is now at an elevated temperatureabove the temperature of stream 8. The temperature of stream 27 isgenerally about 10 C. to 50 C. higher than the temperature of stream 8,and this factor provides the advantages of the present invention asdiscussed supra. Stream 27 is passed into unit 9 below partition 18, andthe process gas flows downwards through catalyst bed 28, which isgenerally similar in configuration and function to bed 10 describedsupra, except that bed 28 is of much reduced size or quantity ofcatalyst as compared to bed 10, for the reasons discussed supra.Additional conversion of sulfur dioxide to sulfur trioxide takes placein bed 28, with resultant temperature rise. The heated gas stream flowsdownwards from bed 28 and in contact with heat exchange coil 29, throughwhich a cooling fluid such as process air or water is circulated. Insome instances, pre-dried process air is circulated through coil 29, andthe resulting heated air is employed as combustion air in furnace 3. Inother instances, boiler feed or condensate water may be circulatedthrough coil 29, with the resultant generation of usable steam. In anycase, the process gas is cooled by coil 29, preferably to a temperaturewhich is about 10 C. to 50 C. above the temperature of stream 8, and thecooled process gas next flows through the final catalyst bed 30. Bed 30is of substantially diminished catalyst volume compared to bed 10, dueto a higher gas inlet temperature as discussed supra. Final andsubstantially complete conversion of sulfur dioxide to sulfur trioxidetakes place in bed 30, and the resultant process gas stream 31discharged from unit 9 below bed 30 is now sub-- stantially free ofsulfur dioxide.

Stream 31 now principally contains sulfur trioxide, residual oxygen andinerts, and will also contain a very minor residual proportion or traceof sulfur dioxide. Stream 31 is produced at a temperature generally inthe range of 420 C. to 550 C., and stream 31 is cooled in waste heatsteam boiler or economizer 32, by indirect heat exchange with boilerfeed or condensate water, prior to final absorption of sulfur trioxide.Water stream 33 is circulated in boiler 32 external to the boiler tubes,and generated saturated steam or mixture of steam and water is removedvia stream 34. The cooled process gas stream 35, now at a temperaturegenerally in the range of 50 C. to 150 C., is passed into gas scrubbingtower 36 below gasliquid contact section 37, which is generally similarin configuration and function to section 23 described supra.Concentrated liquid sulfuric acid stream 38, which generally contains inthe range of about 93 percent to 99 percent sulfuric acid content byweight, is sprayed or otherwise dispersed into unit 36 above section 37,and flows downwards through section 37 countercurrent to the rising gasphase, thereby absorbing sulfur trioxide and forming further sulfuricacid solution by reaction between sulfur trioxide and water. Theresulting concentrated liquid sulfuric acid phase is removed from thebottom of unit 36 via stream 39, and a portion of stream 39 may becooled, diluted with water or dilute sulfuric acid, and recycled viastream 38. A final tail gas stream 40 substantially free of sulfuroxides is discharged from unit 36 above section 37, and in mostinstances stream 40 will be passed to a stack for atmospheric discharge.In instances when the process is operated at elevated pressure, stream40 may be expanded through a gas turbine or the like to recover usablepower.

Referring now to FIG. 2, one embodiment of an application of the processof the present invention to an existing or original once-throughsulfuric acid plant is illustrated, in which reheat of the tail gas fromthe existing or original plant is accomplished in part by heat exchangeof the tail gas with the hot effluent gas from the existing first stageof catalysis in an original plant heat exchanger. The basic chemicalreactions and process conditions of the arrangement of FIG. 2 aregenerally comparable to the process of FIG. 1, and therefore the processsequence of FIG. 2 will only be described in general terms, except inportions of the sequence which vary from the FIG. 1 flowsheet. Sulfurstream 41 is burned with predried combustion air stream 42 in sulfurburning furnace 43 to form hot sulfur combustion effluent stream 44which contains sulfur dioxide, oxygen and inerts such as nitrogen.Stream 44 is split into main portion 45 and bypass portion 46, whichbypasses waste heat steam boiler 47 for temperature control. Stream 45passes through unit 47 in heat exchange with preheated water orwater-steam mixture stream 48, which is vaporized in unit 47 to formsteam stream 49. A cooled process gas stream 50 is discharged from unit47 and combined with hot bypass stream 46 to form stream 51 at atemperature generally in the range of 400 C. to 450 C.

Steam 51 is passed into converter 52 and through the initial catalystbed 53, with resultant conversion of a portion of the sulfur dioxide inthe gas phase to sulfur trioxide. The resultant hot partially convertedgas stream, now at a temperature typically in the range of 500 C. to 630C., is diverted by partition 54 and flows via stream 55 throughgas-to-gas heat exchanger 56, in which the gas stream is cooled byindirect heat exchange with process tail gas, as will appear infra. Thecooled intermediate process gas stream 57 discharged from unit 56 is nowat a temperature typically in the range of 400 C. to 450 C, and stream57 is suitable for further catalytic oxidation of sulfur dioxide tosulfur trioxide. Stream 57 is passed into converter 52 below partition54, and flows downwards through catalyst bed 58. The heated gas formedby the catalytic oxidation in bed 58 is cooled by coil 59, and the gasnext flows downwards through catalyst bed 60. The converted gas stream61 removed from converter 52 below bed 60 is now at a temperaturetypically in the range of 420 C. to 550 C., and stream 61 containssulfur trioxide, residual oxygen, inerts, and a minor residualproportion of sulfur dioxide.

Stream 61 is passed through economizer 62, which is basically a boilerfeed or condensate water preheater in which the process gas stream iscooled prior to scrubbing for sulfur trioxide removal. Boiler feed orcondensate water stream 63 is passed into unit 62 and is heated, withthe resulting heated or partially vaporized water stream 48 beingutilized as described supra. The resulting cooled process gas stream 64discharged from unit 62 is passed into absorption tower 65 belowgasliquid contact section 66, and sulfuric acid or oleum stream 67 issprayed downwards into section 66 and absorbs sulfur trioxide from thegas phase, with resultant concentrated sulfuric acid or oleum beingremoved via stream 68. The residual gas phase stream 69 removed fromunit 65 above section 66 would be discharged to the atmosphere as tailgas in a conventional once-through plant consisting only of units 43,47, 52, 56, 62 and 65.

In this embodiment of the invention, stream 69 is not discharged to theatmosphere, and increased sulfur dioxide recovery as sulfuric acid isattained together with diminished air pollution, by additionalprocessing of stream 69 with added process units as will appear infra.Stream 69 is pressurized by circulating gas blower 70, and the gasstream 71 discharged from blower 70 is heated from an initialtemperature generally in the range of 50 C. to 150 C. to an intermediatetemperature typically in the range of 100 C. to 200 C. in gas-to-gasheat exchanger 72. The heated intermediate temperature gas stream 73discharged from unit 72 is passed through heat exchanger 56 and isfurther heated, and the resulting hot process gas stream 74 dischargedfrom unit 56 is now at an elevated temperature suitable for finalcatalytic conversion of sulfur dioxide to sulfur trioxide. In accordancewith the present invention, stream 74 is at a higher temperature thanstream 51, and stream 74 is typically produced at a temperature which isabout C. to 50 C. higher than the temperature of stream 51.

Stream 74 is passed into secondary converter 75, and final andsubstantially complete catalytic oxidation of sulfur dioxide to sulfurtrioxide takes place in catalyst bed 76. The resulting hot process gasstream 77 discharged from unit 75 now contains sulfur trioxide, residualoxygen and inerts, and stream 77 is substantially free of sulfurdioxide. Stream 77 is passed through heat exchanger 72, and theresulting cooled gas stream 78 is passed into scrubbing tower 79 belowgasliquid contact section 80. Concentrated aqueous sulfuric acid stream81 is sprayed or otherwise dispersed into unit 79 above section 80, andflows downwards through section 80 and absorbs sulfur trioxide from thegas phase. The resulting sulfuric acid stream 82 removed from the bottomof unit 79 is fortified with absorbed sulfur trioxide, which formsadditional sulfuric acid in solution. The final tail gas stream 83discharged from unit 79 above section 80 is substantially free of sulfuroxides, and stream 83 may now be safely discharged to atmosphere via astack or the like, without causing air pollution. In instances when theprocess is operated at elevated pressure, stream 83 may be expandedthrough a gas turbine or other mechanical power recovery means in orderto recover power prior to atmospheric discharge.

Referring now to FIG. 3, an alternative embodiment of an application ofthe process of the present invention to an existing or originalonce-through sulfuric acid plant is illustrated, in which reheat of thetail gas from the existing or originalplant is accomplished in part bydirect injection of hot sulfur combustion furnace effluent gas into thetail gas stream. The basic chemical reactions and process conditions ofthe arrangement of FIG. 3 are generally comparable to the processes ofFIGS. 1 and 2, and therefore the process sequence of FIG. 3 will only bedescribed in general terms, except in portions of the sequence whichvary from the prior FIGS. 1 and 2 flowsheets. Sulfur stream 84 is burnedwith predried combustion air stream 85 in sulfur burning furnace 86 toform hot sulfur combustion effluent stream 87 at a temperature typicallyin the range of 800 C. to l,000 C. Stream 87 usually contains about 8percent to 14 percent sulfur dioxide content by volume, together withexcess oxygen and inerts such as nitrogen. Stream 87 is split intobypass portion 88, which is utilized for tail gas reheat in accordancewith the present invention, and

the main process gas portion stream 89, which is cooled in waste heatsteam boiler 90. Hot boiler feed or condensate water stream 91 is passedinto unit 90 and generated saturated or superheated steam stream 92 isremoved from unit 90 and passed to any various stream usages, such 'asfor process stream heating in ancillary facilities, or for feed steamturbine drives for process pumps or blowers.

The cooled process gas stream 93 discharged from unit 90 is now at atemperature typically in the range of 400 C. to 450 C., and is suitablefor initial catalysis in the original plant converter. Stream 93 passesinto the top of converter 94 and flows downwards through the uppermostcatalyst bed 131. The resulting hot partially converted. gas streamdischarged downwards from bed 131 is diverted by partition 95 andremoved from unit 94 via stream 96, which flows through waste heat steamboiler 97. Hot boiler feed or condensate water stream 98 is passed intounit 97 in indirect heat exchange with the hot process gas, andgenerated steam is removed via stream 99 and passed to usages asdescribes supra. The cooled process gas produced by unit 97 isdischarged via stream 100, which is now at a temperature typically inthe range of 400 C. to 450 C. Stream 100 is passed into unit 94 belowpartition 95, and flows downwards through catalyst bed 101. Theresulting heated process gas is cooled by coil 102 and flows throughlower catalyst bed 103, which is the final catalyst bed of the originalconverter 94. The hot converted gas is removed from the bottom of unit94 via stream 104, which contains sulfur trioxide, residual oxygen andinerts, and a minor residual proportion of sulfur dioxide.

Stream 104 is cooled in economizer 105, by indirect heat exchange withboiler feed or condensate water stream 106 which is circulated throughunit and removed via stream 107 as heated or partially vaporized water.Stream 107 is divided into streams 91 and 98. The cooled process gas isremoved from unit 105 via stream 108, which is passed into gas scrubbingand sulfur trioxide absorption tower 109 below gas-liquid contactsection 110. Concentrated sulfuric acid or oleum stream 111 is sprayedor otherwise passed into unit 109 above section 1 10, and flowsdownwards through section 1 l0 and absorbs sulfur trioxide, withresultant formation of further sulfuric acid or fortified oleum in theliquid phase, which is removed from the bottom of unit 109 via stream112. The residual scrubbed gas phase removed from unit 109 above sectionas stream 113 would have previously constituted a tail gas in theexisting process facility consisting of units 86, 90, 94, 97, 105 and109, however stream 113 is further processed in accordance with thepresent invention as will appear infra, in order to recover residualsulfur dioxide in an improved manner as sulfuric acid.

Stream 113 is pressurized by gas circulation blower 114, and dischargedvia stream 1 15, which flows through gas-to-gas heat exchanger 116 forheating from an initial'temperature typically in the range of 50 C. to150 C. to an intermediate elevated temperature typically in the range of100 C. to 200 C. The resulting heated intermediate temperature gasstream 117 discharged from unit 116 is divided into a minor quenchportion 118 and a major portion 119, which is combined with hot gasstream 88 to form a combined gas stream at a suitable temperature quenchfurther catalysis. In accordance with the present invention, theaddition of hot gas stream 88 to stream 119 heats stream 119 and formsstream 120 at a temperature which is generally above the temperature ofstream 93, and the temperature of stream 120 is specifically 10 C. to 50C. above the temperature of stream 93. Stream 120 flows into thesecondary converter 121 and downwards through the upper catalyst bed122, with resultant conversion of a major portion of the sulfur dioxidecontent of stream 120 to sulfur trioxide. The resultant gas streamdischarged below bed 122 at an elevated temperature is quenched to alower temperature suitable for further catalysis by the addition ofcolder quench gas stream 118, which mixes with the hot gas below bed 122and lowers the overall gas temperature to a level preferably above thetemperature of stream 93 and typically 10 C. to 50 C. above thetemperature of stream 93. The combined gas stream flows downwardsthrough bed 123, in which final and substantially complete catalyticoxidation of sulfur dioxide to sulfur trioxide is attained. The hotfully converted gas stream 124 discharged from unit 1.21 below bed 123is cooled in heat exchanger 1 16, and the cooled fully converted gasstream 125 discharged from unit 1 16 passes into sulfur trioxideabsorption tower 126 below gas-liquid contact section 127. Concentratedsulfuric acid stream 128 is sprayed into unit 126 above section 127 andflows downwards in contact with the rising gas phase, thereby absorbingsulfur trioxide and forming further sulfuric acid in solution. Theliquid phase consisting of further concentrated sulfuric acid dischargedbelow section 127 is removed from unit 126 above section 127, and stream130 is substantially free of sulfur oxides and is suitable for dischargeto the atmosphere via a stack or the like. In instances when the processis operated at elevated pressure, stream 130 may be expanded through agas turbine or other mechanical power recovery means prior to dischargeto the atmosphere.

Numerous alternatives within the scope of the present invention, besidesthose-mentioned supra, will occur to those skilled in the art. Theranges of process variables such as temperature and concentrations ofcomponents in process streams constitute preferred embodiments of theinvention for optimum utilization of the process, and the invention maybe practiced outside of these ranges in suitable instances, except thatthe residual gas stream after primary absorption will generally beheated to a temperature which is about 10 C. to 50 C. above thetemperature of the initial gas stream passed to the first stage ofcatalysis, prior to passing the residual gas stream through the finalcatalytic stage or stages. The invention is generally applicable to asingle stage or a plurality of stages of catalysis prior to intermediateor primary absorption, and a single stage or a plurality of stages ofcatalysis after primary absorption, however as discussed supra optimumresults are attained with two stages before and two stages after primaryabsorption. The initial sulfur dioxide containing process gas stream maybe derived from any or a combination of diverse sulfur sources orsulfur-containing materials besides elemental sulfur per se, such assludge acid derived from chemical processing or petroleum refining,pyrites or sulfide minerals, hydrogen sulfide gas or the like. Ininstances when such raw materials as sludge acid or hydrogen sulfide areburned or otherwise oxidized to yield a gas stream containing sulfurdioxide, this gas stream must be cooled and scrubbed with concentratedsulfuric acid in order to remove water vapor which is concomitantlyformed during combustion, prior to passing the process gas stream to thefirst stage of catalysis, in order to prevent the formation of asulfuric acid mist during catalysis. In some instances, the hot sulfurcombustion effluent gas stream may be cooled in steam boilers such asunits 5, 47 or 90 to a final temperature below the optimum temperaturefor initial catalysis. In this case, a hot gas bypass may be provided,as for example stream 46 in FIG. 2, or the cooled process gas may bereheated prior to initial catalysis by indirect heat exchange with thehot effluent gas from the initial stage of catalysis. In the embodimentof the invention shown in FIG. 2, the heat exchanger 56 when originallyoperated in the existing plant will usually have been a steam boiler orsteam generating facility, and the modification of the existing facilityto dual absorption will reduce this steam production, since process gasstream 73 is not passed through unit 56 in indirect heat exchange,rather than boiler feed or condensate water.

An example of an application of the concepts the present invention tothe design of a typical industrial size facility will now be described.

EXAMPLE The composition of the tail gas that would result from buming100 mols of sulfur in air to form a gas containing 10 percent sulfurdioxide, treating this gas in a conventional contact plant converter soas to convert 91 percent of the sulfur dioxide to sulfur trioxide andabsorbing the sulfur trioxide so formed is:

Sulfur dioxide 9.0 mols/hour Oxygen 64.5 mols/hour Nitrogen 790.0mols/hour If this tall gas is treated in a two-stage converter withmeans for cooling the gas between stages in such a manner that percentof the sulfur dioxide is converted to sulfur trioxide in the first stageand an additional 50 percent of the residual sulfur dioxide is convertedin the second stage, so that of the original mols of sulfur fed to theplant, all but 0.45 mols are converted to sulfur trioxide, then thequantity of catalyst required in this second converter will depend onthe gas temperature at the inlet to each stage asfollows:

Temperature at inlet to each stage. C 429 454 Liters of catal st Stage I4,670 3,050 Liters of catalyst Stage 2 3,560 [.030 Liters of catalystTotal catalyst 8,230 4,080

It is clearly evident that substantially less catalyst is required atthe higher inlet temperature of 454 C. The actual quantities of catalystin a specific instance will depend on the actual catalyst used. In thecomparison data presented supra, the proposed catalyst is vanadiumpentoxide deposited on and mixed with diatomaceous earth and othermaterials.

conventionally, a double absorption sulfuric acid plant has three stagesbefore the primary absorption stage and one stage after. It has beendetermined that, by employing the temperature effect of the presentinvention, it is preferable to have two stages before the primaryabsorber and two stages after. Thus, taking the same gas as describedabove and containing I00 mols sulfur dioxide, llO mols oxygen'and 790mols nitrogen, the catalyst quantities required for the two methods areas follows:

Conventional New High Temperature Dual Absorption Dual Absorption InletTemp. Liters of Inlet Temp. Liters of C Catalyst C Catalyst lst Stage427 2,800 427 2,800 2nd Stage 432 5,620 432 5,620 3rd Stage 432 8,850454 3,050 4m Stage 430 4,300 454 1,030

Total catalyst 21,570 12,500

In the conventional case, the first or primary absorber is after Stage3, and in the new high temperature case according to the presentinvention, the first absorber is after Stage 2. It is clearly evidentthat substantially less catalyst is required in the new scheme, due tothe provision of higher inlet gas temperatures in the 3rd and 4thStages.

If the maximum practicable conversion is required, the gas temperatureinto the last stage of the new process of the present invention as shownsupra may be reduced to 432 C. when with 4,000 liters of catalyst, anoverall conversion of 99.75 percent may be reached. The total catalystis still only 15,600 liters, which is appreciable less than in theconventional case which gives only 99.5 percent conversion.

We claim:

1. In a process for producing sulfuric acid in which an initial hotprocess gas stream containing sulfur dioxide and oxygen is passedthrough a first plurality of catalyst beds at elevated temperature,whereby a portion of the sulfur dioxide content of said gas stream iscatalytically oxidized to sulfur trioxide in each of said firstplurality of beds, said gas stream being cooled after each bed, thecooled gas stream containing sulfur trioxide, residual sulfur dioxideand residual oxygen produced after the last of said first plurality ofbeds is scrubbed with a first stream of concentrated sulfuric acid,whereby sulfur trioxide is removed from the gas stream and a scrubbedintermediate gas stream containing residual sulfur dioxide and oxygen isproduced, said intermediate gas stream is heated, the heatedintermediate gas stream is passed through a second plurality of catalystbeds at elevated temperature, whereby a portion of the residual sulfurdioxide content of said intermediate gas stream is catalyticallyoxidized to sulfur trioxide in each of said second plurality of beds,said gas stream being cooled after each bed, and the cooled gas streamcontaining sulfur trioxide, residual sulfur dioxide and residual oxygenproduced after the last of said second plurality of beds is scrubbedwith a second stream of concentrated sulfuric acid, whereby sulfurtrioxide is removed from the gas stream and a scrubbed final gas streamhaving low residual sulfur dioxide content is produced, the improvementwhich comprises a. heating said intermediate gas stream by indirect heatexchange with the hot gas stream discharged from the last of said secondplurality of catalyst beds, whereby said hot gas stream is cooled,

b. dividing the heated intermediate gas stream formed by step (a) into afirst portion and a second portion,

0. adding a hot sulfur combustion effluent gas stream to the firstportion of said intermediate gas stream, whereby said first portion isheated and a combined gas stream is formed at an elevated temperaturewhich is about C. to 50 C. higher than the temperature of said initialhot process gas stream,

d. passing said combined gas stream into the first of said secondplurality of catalyst beds, and

e. adding said second portion of said intermediate gas stream to thepartially reacted gas stream between successive catalyst beds of saidsecond plurality of catalyst beds, whereby said partially reacted gasstream is cooled to a temperature which is about 10 C. to 50 C. higherthan the temperature of said initial hot process gas stream.

2. The process of claim 1, in which the number of catalyst beds in saidsecond plurality of catalyst beds is two.

3. The process of claim 1, in which said initial hot process gas streamis at a temperature in the range of 400 C. to 450 C., prior to passingthrough said first plurality of catalyst beds.

4. The process of claim 1, in which said first and second streams ofconcentrated sulfuric acid contain in the range of about 93 to 99percent sulfuric acid content by weight.

5. The process of claim 1, in which said first stream of concentratedsulfuric acid is oleum.

6; The process of claim 1, in which said initial hot process gas streamis produced by burning sulfur with predried air to form a combustionefiluent gas stream, and said combustion effluent gas stream is cooledto a temperature in the range of 400 C. to 450 C. to fonn said initialhot process gas stream.

2. The process of claim 1, in which the number of catalyst beds in saidsecond plurality of catalyst beds is two.
 3. The process of claim 1, inwhich said initial hot process gas stream is at a temperature in therange of 400* C. to 450* C., prior to passing through said firstplurality of catalyst beds.
 4. The process of claim 1, in which saidfirst and second streams of concentrated sulfuric acid contain in therange of about 93 to 99 percent sulfuric acid content by weight.
 5. Theprocess of claim 1, in which said first stream of concentrated sulfuricacid is oleum.
 6. The process of claim 1, in which said initial hotprocess gas stream is produced by burning sulfur with predried air toform a combustion effluent gas stream, and said combustion effluent gasstream is cooled to a temperature in the range of 400* C. to 450* C. toform said initial hot process gas stream.